1. Field of the Invention
The present invention relates to a seismic profiling tool and method for measuring characteristics of subsurface formations adjacent a borehole.
2. Description of the Related Art
Seismic imaging of complex geologic structures, such as salt flanks, faults, or steeply dipping beds has required the collection of many tens or even hundreds of downhole source/receiver data pairs from a variety of source/receiver spacing intervals. Collecting such a large volume of data has been expensive and devices used for single well imaging have been difficult and risky to deploy. Known techniques have had inherent inaccuracies and imposed significant opportunity costs.
Deploying hundreds of receiver stations or sources downhole simultaneously has proven to be operationally infeasible. The use of a large quantity of wall-clamped geophone receivers proved particularly impractical because the risk of a receiver string becoming stuck in a well increased with every additional receiver clamped to the borehole wall.
One known technique for seismic imaging involved positioning a small number of receivers in the well and repeatedly acquiring data over the same region on sequential passes while varying the spacing between the source and receivers on each pass. An extensive amount of time was required to repeatedly trip the tool out of the well, during which operations such as drilling or production of hydrocarbons could not be performed. The opportunity cost proved prohibitive.
One known technique for seismic attenuation measurements used vertical seismic profiling data collected at seismic frequencies. For accurate measurements, numerous receivers were required at varying distances from the source for each firing of the source. With numerous receivers being used, there were uncertainties in determining attenuation. A first cause was variations in the characteristics of receiver response between different receivers. Another cause was inconsistent borehole coupling of the several receivers to the borehole wall.